I'm trying to make a sort of Keyless Go style system for my 1996 Mazda Miata using an Arduino and RFID shield as a DIY project for fun. I want to make the Arduino crank the starter until the engine starts, like a human would. However, I need some way for it to know the engine has started. Ideally, I would like some wire which produces a signal only when the engine is running. Is there such a wire? If not, do I need to read the tach signal? Could I check the voltage coming out of the alternator, or would that have voltage when cranking too? Help is greatly appreciated.
Edit: This leads to my second question. What kind of signal does the 1996 Miata's tach take? I understand that it's likely a square wave. What is the voltage? Does it vary the frequency or the duty cycle?